Fix PID arg handling for case where there is only one instance.
Fix PID arg handling for case where there is only one instance.

On a platform where no PID of any kind can be obtained, the daemon
will not permit starting more than one instance, but the commands
(e.g. stop, signal) still take a PID.  Passing 0 as the PID string
will reference the single instance for those cases.  Refs #613

svartika/ccnx,cawka/ndnx,ebollens/ccnmp,ebollens/ccnmp,cawka/ndnx,svartika/ccnx,ebollens/ccnmp,svartika/ccnx,svartika/ccnx,svartika/ccnx,svartika/ccnx,cawka/ndnx,ebollens/ccnmp,svartika/ccnx,cawka/ndnx,cawka/ndnx